Linear actuators wherein a motor rotates an output shaft in response to a voltage input signal, and thereby controls displacement of a linear displacement output shaft are known in the art. In conventional linear actuators, the linear displacement output shaft and motor are coupled to one another by means of a mechanical gear train mechanism. This arrangement is satisfactory for many applications, but reliability and high precision are limited as a result of backlash and hysteresis inherent in the mechanical drive mechanism. Hysteresis and backlash also limit the response time and the high speed capabilities of conventional linear actuators.
Prior art linear actuators are typically arranged with the motor and the linear displacement output shaft positioned end-to-end with respect to one another. This configuration results in a linear actuator of significant length. The awkward dimensions of conventional linear actuators complicate installation, and may render the actuators unsuitable for use in certain applications.
Conventional linear actuators feature open loop control. When voltage input signals are communicated to the motor in an open loop control system, corresponding linear displacements are theoretically achieved each time, but there is no feedback system for monitoring and verifying that result, and correcting it if necessary. As a result of the hysteresis and backlash effects prevalent in conventional linear actuators, input drive voltages frequently do not translate perfectly into linear displacement of the output shaft.
To compensate for linear displacement errors induced by hysteresis and backlash, conventional linear actuators designed for high precision applications employ programmed position overshoots. In other words, the linear actuators are extensively and finely calibrated to determine the positive or negative linear displacement error resulting when specified input voltages are communicated to the motor. The voltage input is adjusted to compensate for the anticipated linear displacement error, and the adjusted input voltage is communicated to the motor. The calibration system is complex and expensive, and it has not generally resulted in the desired high degree of precision.
Accordingly, it is an object of the present invention to provide a high precision linear actuator that is not subject to hysteresis and backlash effects. It is a further object of the present invention to provide a linear actuator capable of precise low speed and high speed bidirectional linear displacements. It is also an object of the present invention to provide a linear actuator having a closed loop control system providing a high degree of reliability, resolution and precision, without requiring extensive calibration